The actin cytoskeleton is adapted to many different structures and involved in diverse cellular functions. Differentiation of the cytoskeletal structures is generally controlled by a number of actin-binding proteins that regulate dynamics and organization of the actin filaments. Myofibrils in striated muscle are highly differentiated forms of the actin cytoskeleton and specialized for generating contractile forces. However, the mechanisms by which actin filaments are assembled into myofibrils during development and maintained in the contractile apparatuses are largely unknown. The nematode Caenorhabditis elegans has obliquely striated muscle in the body wall, which shares a number of biochemical and functional similarities with vertebrate striated muscle. We have identified actin depolymerizing factor (ADF)/cofilin, actin-interacting protein 1 (AIP1), and tropomyosin as key regulators of actin organization in the body wall muscle. When these proteins are defective, the actin filaments are disorganized and the contractility of the body wall muscle is impaired. In addition, we obtained evidence that UNC-87, a calponin-like protein, and kettin, immunoglobulin-like repeat protein, may stabilize actin filaments, and PFN-3, a muscle-specific profilin, may help recycling actin monomers for polymerization. Although genetic and biochemical analyses of these proteins suggest that each of these actin-binding proteins might be important for regulation of actin dynamics in muscle cells, we do not understand how these proteins functionally interact. We hypothesize that ADF/cofilin is a key enhancer of actin dynamics and functionally interacts with other actin-binding proteins to regulate actin polymerization, depolymerization, and stability during assembly and maintenance of the striated myofibrils. In this project, we aim to investigate mechanisms of three aspects of actin dynamics in muscle cells in C. elegans: (Aim 1) To determine how ADF/cofilin and AIP1 disassemble actin filaments in muscle cells, (Aim 2) To determine how actin monomers are recycled for polymerization in muscle cells, and (Aim 3) To determine how actin filaments are stabilized in muscle cells. These studies should provide important information not only on the mechanism of normal myofibril assembly and maintenance but also on the pathogenesis of nemaline myopathy in which misregulation of actin filament stability is implicated.